In many potentiometric position-sensing applications, sliding electrical contacts must perform reliably in harsh environments and must be resistant to mechanical wear and also contribute little or no electrical noise over the lifetime of the device. Since the useful lifetime of these low current, low voltage devices is often measured in terms of millions to tens of millions of full cycle rotations and tens to hundreds of millions of dither cycles, the contacts are often made from relatively high cost, high strength alloys containing mixtures of noble metals, such as gold, platinum, and palladium. Since noble metals by themselves do not have sufficient strength for these demanding applications, they are often alloyed with moderate levels of elements such as copper and silver, which provide strength but allow the alloys to maintain the excellent tarnish and oxidation resistance typical of the noble metals. Examples of these demanding applications include automotive fuel level senders, automotive throttle position sensors, and exhaust gas recirculation (EGR valve) sensors. These applications often require exposure to corrosive environments and/or to elevated temperatures. U.S. Pat. Nos. 5,833,774 and 5,484,569 to Klein et. al. disclose compositions of silverl/palladium/copper alloys which are used in such applications.
The cost of these alloys is directly related to the relative amounts of noble elements contained in the alloy. Until recently, palladium was the least costly of the noble metals with a cost roughly one third to one half that of gold and platinum. For that reason, palladium-based alloys such as PALINEY.RTM. 6 and PALINEY.RTM. 7, manufactured by the J. M. Ney Company, were widely used in high reliability potentiometric applications. However, since palladium-based alloys were still much more costly than alloys based on copper or nickel, designers were often forced to reduce the size of palladium-based alloy contacts to remain cost-competitive. In recent years, the cost of palladium has risen so dramatically that it is now roughly equivalent to platinum and more costly than gold. Because of this dramatic increase, there is now a demand for sliding electrical contact materials with reduced palladium levels without compromising the strength and hardness typical of alloys having greater amounts of palladium.
For many of the miniaturized sliding contact applications, the alloy is used as both a cantilever-type spring member and as a low electrical noise contact. The force of a cantilever spring is proportional to the elastic modulus, and limited by the yield strength, of the material used for the contact. A spring member is used to maintain electrical continuity across the sliding contact interface. Any disruption of that continuity results in an electrical spike, or electrical noise. For low yield strength materials, the spring force is maintained by increasing the cross sectional area of the spring member. As the cost of the alloy used for the spring member increases, designers look for lower cost, higher strength materials to perform this function.
Commercially available copper-nickel-zinc alloys are well known and are frequently referred to as "nickel silver" alloys because of their white or silvery color, even though they contain no silver. These alloys are typically used for hardware, optical parts, jewelry, mechanical springs, and sliding contact applications in which electrical noise and wear are not considered detrimental. Although they are relatively inexpensive, their suitability for use as high-precision miniature sliding potentiometric contacts is limited by their relatively low strength and hardness, poor tarnish resistance, and tendency to wear. In sliding applications, the resultant wear debris can be a source of electrical noise. Additionally, the prior art "nickel silver" alloys are known to have poor stress relaxation characteristics, and thus their utility in elevated temperature applications is limited. In order to be used as a cantilever spring contact, the prior art "nickel silver" alloys require relatively large cross sectional areas to carry a given load. This large size requirement, along with their relatively poor wear resistance, poor stress relaxation, and tendency to produce wear debris and create electrical noise, severely limits the use of these alloys as sliding contacts in precision potentiometric sensors.
The prior art commercially available copper-nickel-zinc alloys are single phase, solid solution alloys that can only be strengthened by cold working the alloy after it has been cast. As a wrought (i.e., cold worked only) material, these alloys have limited ductility in the harder tempers. In general, for wrought alloys, ductility decreases as the tensile strength increases. Additionally, for these alloys, the only way to regain ductility is to heat or anneal the alloy, which reduces the strength of the alloy.
In contrast to the strengthening mechanism of cold working, age hardening as a strengthening mechanism provides an alloy having both high strength and sufficient ductility to allow for complex forming operations. For age hardening to occur, the alloy should be formulated to have a stable multi-phase microstructure at low (ambient) temperatures. In addition, one or more of the phases should go into solution at an elevated temperature.
Age hardening involves heating an alloy to a temperature at which at least some of the solute elements are in solid solution, and then cooling the alloy sufficiently quickly to ensure that the solutes remain in solution, followed by re-heating at an intermediate temperature. During this re-heating or aging step, certain of the constituent elements will precipitate to form a second phase within the matrix. The precipitated phase can have a different crystal structure from that of the matrix phase. During aging, the hardness and yield strength of the alloy increase to a maximum value when the precipitated phase reaches an optimum size and distribution. In contrast to wrought-only strengthened alloys, the aging process can also increase ductility while it strengthens the cold-worked alloy. Age hardening can also increase the strength of annealed (solutionized) alloys while maintaining good ductility. These properties are of primary interest in alloys that are used for miniature, cantilever beam--type sliding electrical contacts.
Prior art "nickel-silver" alloys are not known to be age hardenable. They achieve only moderate strength and hardness levels, low elongation, and relatively poor ductility typical of a wrought alloy. Additionally, the "nickel-silver" alloys are not known to be particularly resistant to elevated temperature stress relaxation, environmental tarnish, or oxidation. All of these are important properties for miniature sliding contacts used in automotive and other demanding position sensor applications.
It would therefore be advantageous to provide a copper-based alloy for use in sliding, static and other electrical contact applications requiring high strength. It would be fiurther advantageous if superior mechanical properties could be achieved with a material of a lower unit cost than the prior art silver-palladium alloys.